I would like to know how to use dyntopo with multi-density
1. Avoid going too hi-poly if you don't have to. If you're planning to sculpt at this level of detail on your whole mesh, make sure your computer can keep up. And ask yourself if you really need that much geometry. In my opinion if your wireframe turns black in some spots, that's too many polygons.



You can change these settings to control how fine your sculpt will be. Personally, I find that Constant Detail is easier to manage than Relative Detail.



2. The Smooth operation can help.

Back to basics, just press W in Edit mode and choose Smooth from the Specials Menu. If you Grow Selection (Ctrl+) and repeat, you can smooth over more area in steps.

3. You can always retopo (and usually should retopo after Dyntopo).

Personally, I like to do a Multires retopo to preserve the high level of detail but also be able to still manage the mesh.